1. Field of the Invention
The invention relates to ball-and-socket joints and in particular to prosthetic joints to be used as implanted hip joints.
2. Description of the Prior Art
Many kinds of prosthetic ball-and-socket bearings for artificial joint to be implanted in human body are known in the art. Most of the presently used joints of this type use a frictional engagement between the ball portion or head of the joint and the socket. Examples of such known joints include those described in U.S. Pat. No. 4,380,090 issued Apr. 19, 1983 (Ramos) where an artificial hip joint has a bearing insert registrable with the first cavity, and a second cavity, having an annular bearing. U.S. Pat. No. 4,801,301 issued Jan. 31, 1989 discloses a prosthetic ball-and-socket joint having a bearing member surrounding a portion of the ball portion or head and rotatable within a spherical cavity. U.S. Pat. No. 5,507,818 issued Apr. 16, 1996 (McLaughlin) discloses a multipolar endoprosthesis including multiple wedge shaped, cylindrical components for adapting the position of the head of the joint. The joint disclosed in U.S. Pat. No. 5,549,697 issued Aug. 27, 1996 (Caldarise) shows a dampening mechanism absorbing some of the loading forces communicated to the acetabular cavity. U.S. Pat. No. 5,549,704 issued Aug. 27, 1996 to Sutter is concerned with a cap forming the ball portion of the joint and its attachment to a bone without a binder.
As mentioned above, the known joints use a frictional engagement, metal-to-metal or metal-to-plastic, between the head and the socket of the joint. Failure of a conventional total hip prosthesis is most often caused by aseptic loosening of either the femoral or acetabular components. Osteolysis (loss of bone) contributes significantly to loosening. When a total hip is loose, the patient's ambulation is dramatically reduced secondary to pain. The patient must undergo revision surgery. Conventional total hip arthroplasty comprises a metal femoral head articulating with a plastic acetabular liner. Wear particles from the ultra high molecular weight polyethylene (UHMWPE) have been implicated in causing osteolysis, loosening and premature failure of total hip joints. As a result, a renewed interest exists in "metal on metal" articulations that do not contain plastic. "Metal on metal" designs have been around since the early seventies, but it is not until recently that there has been renewed scientific interest. With traditional "metal on metal" designs, a metal ball portion articulates with a spherical metal liner. This is a crude "bearing" surface that does in fact produce metallic debris. Metallic debris also contributes to inflammatory tissue responses, probable loosening and thus premature failure and the need for revision surgery.
The above disadvantages have been recognized and mentioned in U.S. Pat. No. 5,556,434 issued Sep. 17, 1996 (Epstein) wherein the three-axis movement emulating the action of a natural hip joint is provided by a universal assembly mounted on a series of roller bearings in a horizontal swivel. While this solution may reduce the formation of undesired debris, it is very complex in structure and requires special bearings having the desired strength while providing a small enough size for accommodation of the entire mechanism within the prosthetic joint structure. Even more important, the joint is inevitably of a constrained type. In other words, the dislocation of the prosthetic joint upon an accident which would merely dislocate the joint cannot occur. As a result, complicated fractures often occur at the femoral or acetabular end, or both, of the implanted joint. These require complex surgical corrections. A mere dislocation of the joint would be much more desirable.
Another known hip joint utilizing a set of bearing balls is described in U.S. Pat. No. 5,092,898 to Bekki et al. In a first group of embodiments, a complex artificial joint head portion is shown having an inner head and an outer head and a rolling ball insert therebetween. The outer surface of the outer head is polished to a mirror for engaging the acetabular roof. These embodiments, when in use, combine a limited angle movement between the inner and outer heads effected by the rolling balls. However, on more substantial angles of movement, a frictional displacement between the outer head and acetabular roof is required. In a second group of embodiments, Bekki et al show a restrained joint where the head of the joint is held in the socket by a retainer ring or assembly which is fixedly secured to the socket, i.e. to the roof of the acetabulum. Thus, any accident which might normally result in a mere dislodgement of the joint would at least destroy the retainer ring and most likely at least a part of the acetabulum, not to mention the release of the race with the rolling balls.